In recent years, a system called a content distribution system has been increasingly put to practical use. In the content distribution system, digital content (hereafter referred to as “content”) such as music, video, and games is distributed from a server apparatus to a terminal apparatus via Internet communication, digital broadcasting, and the like, to enable the content to be used in the terminal apparatus. Typical content distribution systems employ a copyright protection technique to protect a copyright of content and prevent unauthorized use of the content by a malicious user and the like. In detail, the copyright protection technique is a technique for securely controlling use of content by means of a cryptographic technique and so on.
For example, Patent Reference 1 describes the following system as one example of content distribution system. A terminal apparatus receives encrypted content, a usage condition, and a content decryption key from a server apparatus, verifies whether or not tampering has been made, and then verifies whether or not the usage condition is satisfied. Only when all of the verifications are successful, the terminal apparatus decrypts the content.
Thus, in conventional content distribution systems, a license (a generic name for data that includes a usage condition and a content decryption key, also referred to as a “usage right”) is distributed from a server apparatus to a terminal apparatus. Since a public line of the Internet and the like is typically used as a path for distributing the license, there is a need to protect the license from eavesdropping and tampering. That is, there is a need to prevent unauthorized tampering of the usage condition and leakage of the content key. Furthermore, the server apparatus needs to perform authentication of the license distribution destination, in order to avoid distributing the license to an unintended terminal apparatus. A protocol for prevention of eavesdropping/tampering and authentication of a party-in-communication is called a Secure Authenticated Channel (SAC) protocol. For instance, Secure Socket Layer (SSL) is well known (for example, see Non-patent Reference 1).
Moreover, when communication disconnection occurs during license distribution due to a failure of a communication apparatus or a communication line, power discontinuity, and the like, there is a possibility that the license may be lost. In such a case, the user suffers an inconvenience of being unable to reproduce the content which the user purchased. For example, Patent Reference 2 and Patent Reference 3 each describe a protocol for avoiding loss of communication data due to communication disconnection, by means of data retransmission.    Patent Reference 1: Japanese Patent No. 3276021    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2002-251524    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2003-16041    Non-patent Reference 1: A. Frier, P. Karlton, and P. Kocher, “The SSL 3.0 Protocol”, [online], NetScape Communications Corp., Nov. 18, 1996 (searched on Jan. 23, 2006).